1. Field of the Invention
This invention pertains generally to power amplifiers, and more particularly to a zero voltage switching (ZVS) contour based switching power amplifier having a wide dynamic range.
2. Description of Related Art
There are two types of power amplifiers; linear and switching. Linear power amplifiers (PAs), such as class A, B, and AB amplifiers, are biased for peak output power and consequently suffer from poor efficiency at backed-off power levels. A common design approach used to obtain the desired linearity for a PA is to design the PA to handle more power than the level at which it will be operating. This is called “power back-off”, and the differential between design output and operating output is typically expressed in dB. There are also techniques that have improved the efficiency of linear PAs, such as found in transformer combiner based PAs and envelope tracking PAs. However, a transformer combiner based PA is limited by die size constraints, and envelope tracking PAs suffer from supply regulator bandwidth and efficiency problems.
Architectures used in switching PAs, such as supply modulation (e.g., polar, polar loop), theoretically offer high efficiency even at very low output power levels. Yet, these architectures suffer from supply regulator inefficiency, particularly while handling wide bandwidth envelope variations. Recently developed digital PA architectures, such as the digital envelope modulator and the switching mixer PA architectures, all suffer from efficiency degradation at backed-off power levels. Duty cycle modulation and dynamic load modulation, such as used for class-E PAs, can achieve high peak efficiency but results in poor efficiency at low output power levels. In fact, class-E PAs operate sub-optimally at low output power levels when zero voltage switching (ZVS) conditions are not satisfied, thus resulting in significant losses and poor efficiency. It will be appreciated that ZVS is intended to maximize efficient operation of class-E PAs.